Memories presently utilized for the storage of data and other information are normally of two types. First, there are the inexpensive but slow disk, drum and tape memories which may be utilized for storing large quantities of information. With a magnetic disk, for example, as many as 10.sup.9 bits may be stored on a single disk at a cost of about a tenth of a cent per bit. However, random access time with a magnetic disk memory is in the order of 10.sup.4 to 10.sup.5 microseconds.
The second type of memories are the faster but far more costly core and semiconductor memories. Partly as a result of cost, these memories are not normally utilized for storing large quantities of information. Typically, a core memory might store 10.sup.6 bits at a cost of a cent a bit and have a random access time of about one microsecond. Semiconductor memories are normally adapted to store about 10.sup.3 -10.sup.5 bits at a cost of from one to ten cents a bit with a random access time approaching 0.1 microseconds.
From the above it is apparent that there is a wide disparity in cost, speed and capacity between the two different types of memories. Recent developments have suggested that optical memories may provide a vehicle for bridging this gap, providing random access times approaching those of core memories with capacities and per bit costs approaching those of disk memories.
Optical memories have been proposed in which data is permanently stored by laser beam ablation of small discrete regions of a thin film of a metallic storage material, for example a 500 A thick film of bismuth. A disadvantage of the ablation type of optical memory is the high power required of the writing light beam since the writing light beam must have power sufficient to vaporize or melt the discrete regions of the storage material to provide the information indicative holes in the storage material. A factor in the high writing light beam power required for the ablation type of optical memory is that suitable metallic storage materials reflect a large percentage of the laser light beam impinging thereon and that reflected light does not contribute significantly to the melting or vaporization of the discrete regions of the storage material. A further disadvantage of the ablative type of optical memory is that the storage material is not frequency selective during readout and thus the system cannot distinguish between ablations purposely recorded and abrasions and destructions of portions of the storage material inadvertently produced during handling of the storage medium. The lack of frequency selectivity also complicates read during write and other error checking procedures. Furthermore, some storage materials undergo chemical changes in the presence of the atmosphere, for example, the oxidation of bismuth to a substantially non-reflecting oxide of bismuth, thereby making the detection of stored information extremely difficult.
Other types of optical memories utilize storage meterials which can assume two or more physical states in response to applied heat. Since the different physical states will possess different optical properties, these storage materials have been able to function as optical information storage devices when used in the form of a thin film. However, these changes of state optical memories suffer from low readout efficiency and, like ablative type optical storage systems, do not have frequency selectivity and require high power laser beams to effect the change of physical state.